THE CHROMOSOMAL BASIS OF INHERITANCE 9.17 Chromosome behavior accounts for Mendel’s principles Genes are located on chromosomes Their behavior during meiosis accounts for inheritance patterns

9.4 Homologous chromosomes bear the two alleles for each characteristic Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes GENE LOCI DOMINANT allele P a B P a b RECESSIVE allele GENOTYPE: PP aa Bb HOMOZYGOUS for the dominant allele HOMOZYGOUS for the recessive allele HETEROZYGOUS Figure 9.4

The chromosomal basis of Mendel’s principles Figure 9.17

9.18 Genes on the same chromosome tend to be inherited together Certain genes are linked They tend to be inherited together because they reside close together on the same chromosome

Figure 9.18

9.19 Crossing over produces new combinations of alleles This produces gametes with recombinant chromosomes The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

A B a b B A a b A b a B Tetrad Crossing over Gametes Figure 9.19A, B

Figure 9.19C

9.20 Geneticists use crossover data to map genes Crossing over is more likely to occur between genes that are farther apart Recombination frequencies can be used to map the relative positions of genes on chromosomes Chromosome g c l 17% 9% 9.5% Figure 9.20B

SEX CHROMOSOMES AND SEX-LINKED GENES 9.21 Chromosomes determine sex in many species A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

Parents’ diploid cells (male) (female) Parents’ diploid cells X Y Male Sperm Egg Offspring (diploid) Figure 9.21A

Other systems of sex determination exist in other animals and plants The X-O system The Z-W system Chromosome number Figure 9.21B-D

9.22 Sex-linked genes exhibit a unique pattern of inheritance All genes on the sex chromosomes are said to be sex-linked In many organisms, the X chromosome carries many genes unrelated to sex Fruit fly eye color is a sex-linked characteristic Figure 9.22A

Their inheritance pattern reflects the fact that males have one X chromosome and females have two These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait Female Male Female Male Female Male XRXR XrY XRXr XRY XRXr XrY XR XR Xr Xr XR XR XRXr Y XRXR Y XRXr Y Xr Xr XRY XrXR XRY XrXr XRY XrY XrY R = red-eye allele r = white-eye allele Figure 9.22B-D

9.23 Connection: Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Figure 9.23A

Czar Nicholas II of Russia A high incidence of hemophilia has plagued the royal families of Europe Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.23B